V404 Cygni is a binary star system in which there is a black hole. Observations at X-ray and optical wavelengths have shown the system contains a late G or early K star that orbits, every 6.47 days, a compact companion with a probable mass of 8 to 15 solar masses, well above the mass limit at which a collapsed star must become a black hole. Most of the time the X-ray emission is quiescent, indicating only a slow trickle of material from the bright star onto the black hole's accretion disk.

 

 

In 2009, the distance to V404 Cygni was determined accurately by stellar parallax to be 7,800 ± 460 light-years (2,390 ± 140 pc).